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MG MGB Technical - Oil Pressure Gague trouble?

I have the electric oil pressure gague and sending unit on my '70 BGT. When I turn the ignition to the ON position my oil pressure gague climbs to the top, without the engine running? I checked the wire at the sender and have NO continuity..no light. I replaced the voltage stablizer with a new solid state one.Ground wire on gague is in place. I am wondering if this may be a bad gague? any furthur tests I should do?? Thanks
MK Mike

Sounds like the wire from the gauge to the sender is grounded somewhere. Since the gauge is going to full scale it works but may not be accurate. I would try a temporary wire from the sender to the gauge in place of the green/blue wire on the gauge and sender. Be sure to disconnect both ends of the old wire before hooking up the test wire.

Clifton
Clifton Gordon

Clifton. I will give that a try. When this is working correctly I should not see any movement until the engine starts?? correct?
Mike
MK Mike

Mike, That is correct.

Clifton
Clifton Gordon

The voltage stabiliser only fed the oil gauge for the first year of use (1968) after that it was fed with system voltage off a green as the sender contains its own stabiliser. You should see 12v on the white/brown at the sender with the engine stopped, briefly pulsing down towards ground (best seen with an analogue meter) with the engine running, the longer the pulse the higher the pressure. If removing the white/brown from the sender causes the gauge to drop it is the sender which is faulty, if not then *probably* the wire shorting to ground as advised, although it could still be a problem at the gauge.
Paul Hunt 2

Ok, went and checked the power (green) to the gague terminal and it has power. Next checked the output terminal from the gague to the sender (Wn) and I get a faint light- engine not running. When I re-attach the wire from the gague to the sender, NO light at all.on the terminal. So I guess you are correct Clifton. How do I go about hunting down this ground short?? Wires behind the dash look untouched and since this wire runs inside the loom should I just add a new wire or fix this one? Thanks for the help. Mike
MK Mike

Mike to double check it may be a good idea to remove the WN wire from the gauge and be sure the gauge remains at zero with ignition on/engine not running, unless you have already done that. Sorry about giving you the wrong color for the wire on my earlier post, Paul corrected my oversight. I don't recall if there is a bullet connector in the loom under the hood near the fuse box. If there is I would disconnect the wire from the bullet and work on each section separtely. It's so much trouble to unwrap a loom, I would disconnect and tape the old wire ends and run a replacement wire outside the loom and make a note so you can remember why it's there. Use nylon ties to keep it in place.

Clifton
Clifton Gordon

Doesn't appear to be any connections along the run of that wire. You need to check for full 12v on the white/brown gauge terminal with the wire *removed* to make sure it isn't an internal short inside the gauge. Bothers me a bit you got some voltage with the wire disconnected from the sender, but less or none with it reconnected. With the engine stopped the voltage *should* be the same, as I believe it is a pulsed signal from the sender, not a varying resistance.

I'd agree with Clifton that running a replacement wire (if it *is* the wire that is shorting to ground) is much easier than unwrapping the loom and repairing the insulation. But if something *has* chafed through that wire, it may also be attacking other wires in the harness. If these are headlight, ignition, or particularly brown wires you could potentially have a much bigger problem on your hands with a burnt-out harness if not car. Make sure the big grommet in the firewall hole is properly fitted, then it is down to things chafing behind the dash. The engine compartment part of the run is relatively easy to check.
Paul Hunt 2

Thanks guys. I checked gague with WN wire removed and gague doen NOT register. Did as you said Clifton and checked bullet connectors on the harness near fuse box. Now I am getting voltage at the Sender end of the wire, dim light. But when all is connected the gague acts the same, rises to full scale?? I will get the volt meter out and try the terminal to the sending unit? Mike
MK Mike

Well if I'm using this thing correctly, digital hand held multi meter. It settles at about 11.42 Volts at the gague terminal that goes to the sender. Aprox. the same voltage at the end of the wire at the sender.
Mike
MK Mike

Does the gauge read full scale or zero with the white/brown wire removed from the sender? Ditto when the wire is connected to the sender?

If zero when disconnected but full scale when connected then the sender is faulty.

But if the gauge now works as it should with everything connected then you have disturbed the probable short, but it is likely to return. Pulling the harness around near the fusebox maybe what has disturbed it, but there are no bullet connectors there for that circuit. Bullet connectors are only provided to connect one harness or sub-harness to another, not when a wire simply goes between two components like the oil gauge.
Paul Hunt 2

Paul
With the WN wire removed gague registers = 0
WN wire attached, gague = Full
When I first checked the voltage at the Sender Unit with my test light, it was very bright/sunny outside and I may have not seen the dim glow of the bulb, I have a pretty severe visual impairment also so...
I saw there were not any bullets with the wire gague/colours at near the fuse box. So I'm thinking bad sending unit...?
MK Mike

Mike, Sounds like you have found the problem.

Clifton
Clifton Gordon

Guys, Thanks very much for helping me diagnose this glitch. Will be picking up a sending unit this weekend. Will give the Sunpro unit a go since it's affordable. I will keep my eyes open for the hardware to maybe change over to mechanical gague at some point. I will report how the new unit works out. Thanks again. Mike
MK Mike

This thread was discussed between 16/10/2006 and 20/10/2006

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